The invention relates to an optical relay, which transmits light in dependence on the electric field parallel to the direction of propagation of the light toward a target having a first face which is scanned by means of an electron beam. A anode collects the secondary electrons emitted by the beam, and an optically transparent and electrically conducting plate is arranged at the second face of the target, which plate receives the electric video information signal, thereby forming a control electrode. The target is an electrically insulating material which becomes ferroelectric below a certain temperature, called the Curie-temperature, in the proximity of which the optical relay operates.
Such an optical relay for use in television projectors is described in Patent Specifications FR No. 1 473 212 and FR No. 1 479 284. For a better understanding of the invention, the working principle of the optical relay is described below. More extensive information can be obtained from the documents cited.
The invention relates to the conversion of an electric signal which is variable in time and represents the video information, into a visible picture. It is known that this is one of the functions of a television receiver.
In the display tube of such a receiver the electron beam usually performs the three fundamental functions of this conversion:
f1--the beam supplies the energy to be converted into light: the light output power of the tube is therefore always lower than the power transferred by the beam; PA0 f2--the beam scans the surface of the picture; PA0 f3--the beam applies the video information. PA0 piezoelectric coefficients PA0 electrooptical coefficients PA0 dielectric constants .epsilon..sub.x and .epsilon..sub.z. PA0 an effective central area where the target exhibits a single domain ferroelectric phase, and PA0 a neutral peripheral area at a higher temperature than the Curie temperature, where the target exhibits a paraelectric phase.
Due to the functions f2 and f3, amongst others, the power of the beam and hence the image brightness cannot be raised to an extent as is required for projection onto a large screen, for example.
For this reason it has been proposed to separate functions and to have the function f1 performed, for example, by an arc lamp and the functions f2 and f3 by a socalled "optical relay". In this case a crystal is used which produces an electrooptical effect, the so-called "Pockels effect". A crystal of double acid potassium phosphate KH.sub.2 PO.sub.4, hereinafter called KDP, has been found to be suitable.
This effect can be described partially and briefly as follows: when the electrically insulating crystal is subjected to an electric field parallel to its crystal axis c (the three crystal axes a, b and c form a block of three rectangles, in this case, the axis c being the optical axis), the index n of this crystal for light rays in the c direction with linear polarisation in the ab-plane depends upon the direction of said polarisation. To be more precise, if X and Y designate the bisectors of the axes a and b and if the parameters of the crystal with respect to these different directions are designated by the letters used for the directions, the diagram of the indices in the ab-plane forms an ellipse having axes X and Y, instead of forming a circle, and the difference n.sub.x -n.sub.y is proportional to the electric field applied. It follows therefrom that, if the incident light rays are polarised parallel to the axis a, for example, the intensity I of the light passing through an output polariser is I=I.sub.o sin.sup.2 kV if the direction of polarisation of this polariser is parallel to the axis b, and I=I.sub.o cos.sup.2 kV if the direction is parallel to the axis a, wherein I.sub.o is equal to the intensity of the incident light if no parasitic absorption occurs, and wherein V is the electric potential difference between the two planes of the crystal and k is a coefficient depending upon the crystal material used.
In order to obtain a projected picture by means of a lamp by means of said device, it is sufficient, as stated above, to apply an electric field parallel to the axis c and to cause the value of the field at any point of the target to correspond to the brightness at the corresponding point of the picture to be obtained. For this purpose an electron beam emanating from an electron gun and traversing conventional deflection members scans the target, so that the beam performs the function f2. The function f3, here the control of the electric field, is also performed by the beam in the following manner.
When the electron beams strike the surface of the target, they cause,if their energy remains within suitable limits and in so far as the potential of the anode is sufficiently high, an emission of secondary electrons in an amount which is far greater than that of the incident electrons. This results in an increased electric potential of the point struck, so that the difference in potential between the anode and this point decreases. If the electrons of the beam reach this point in a sufficiently large number, the difference in potential becomes negative and reaches such a value (for example, -3 V) that each incident electron emits only one single secondary electron. Thus, the potential of the point is fixed at a limited value with respect to that of the anode. Consequently, taking into account the scanning rate, it suffices if the beam intensity is sufficiently high. As the potential of the anode is constant, each passage of the electron beam fixes, as has been said, the potential of any point A on the struck surface at a value V.sub.O independent of the point and the instant of passage. But the corresponding electric charge produced at the point depends upon the potential of the control electrode situated nearby, at the other side of the target.
If the potential of the electrode at the moment of passage is called VA, the charge is proportional to VO-Va, VA representing the value of the video information signal at the moment of its passage.
The target, whose birefringence depends upon the electric field, is formed by a single crystal of KDP, in which about 95% of the hydrogen is formed by heavy hydrogen (deuterium).
With a given thickness of the crystal, the Pockels effect is proportional to the charges produced on the crystal faces and hence, with a given control-voltage, to the dielectric constant of the crystal. For this reason a target is used of a crystal which becomes ferroelectric beneath a certain temperature, called the Curie temperature, and it is advantageous to use a temperature of a value approaching the Curie temperature because the dielectric constant then attains very high values and the optical relay can function by means of readily obtainable control-voltages (the Pockels effect being proportional to the product .epsilon.V).
The most frequently used crystals exhibiting this phenomenon are acid salts, notably of the KDP type in the class of the quadratic crystals, the optical axis of which is parallel to the crystal axis c. Its Curie temperature is about -53.degree. C. Below the Curie temperature the DKDP is a quadratic crystal, symmetry class 42 m, and it exhibits a paraelectric behaviour. Below the Curie temperature, the DKDP becomes orthorhombic, symmetry class mm2, and it exhibits a ferroelectric behaviour: a locally spontaneous polarisation and the appearance of ferroelectric domains.
At the ambient temperature, the crystal is anisotropic but in the proximity of the Curie point the anisotropy becomes very important. The change of state is accompanied by sudden variations in the physical properties along the crystal axes:
In this way the value of the dielectric constant .epsilon..sub.z changes from approximately 60 at the ambient temperature to 30,000 at the Curie temperature.
It is known that from an electrooptical point of view the apparent thickness e of the crystal of DKDP is EQU e=1(.epsilon..sub.x /.epsilon.'.sub.z) .sup.0.5.
The target appears all the more thin as the ratio .epsilon..sub.x /.epsilon.'.sub.z is smaller, where .epsilon.'.sub.z is the value of .epsilon..sub.z when the crystal is blocked mechanically. As a matter of fact, in an optical relay the monocrystalline plate of DKDP having a thickness 1 close to 250 microns is firmly bonded to a rigid support: a fluorite plate having a thickness of 5 mm.
Consequently, the target of the optical relay is usually cooled to a temperature of approximately -51.degree. C., that is to say a temperature which is slightly higher than the Curie point. Under these conditions .epsilon..sub.x /.epsilon.'.sub.z .perspectiveto.1/9 and the apparent thickness of the crystal is about 80 microns, which provides the optical relay with a good picture resolution. Below the Curie point, the ratio .epsilon..sub.x /.epsilon.'.sub.z is still smaller which substantially improves the picture resolution.
So far it has not been possible to project television pictures by means of a target which is cooled to a temperature below its Curie temperature. As a matter of fact, due to the change of state ferroelectric domains appeared systematically which produced a great number of vertical and horizontal bright lines on the projection screen which are disorderly arranged across the picture. These domains correspond to zones having a different atomic arrangement.